In recent years, the regulation on exhaust gas of motor vehicles has been tightened around the world. In order to cope with this, various exhaust gas-purifying catalysts for purifying hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx) and the like in exhaust gas have been developed. In particular, the European vehicle emission standards EURO6 has strict regulations on NOx emissions. For this reason, an exhaust gas-purifying catalyst exhibiting excellent NOx purification performance is demanded.
In addition, a decrease in fuel consumption of motor vehicles has been increasingly demanded in recent years. In order to meet this demand, a lean-burn engine has attracted attention as an internal combustion engine mounted on a motor vehicle. Moreover, a NOx storage and reduction catalyst (NSR catalyst) has been proposed as an exhaust gas-purifying catalyst for a motor vehicle equipped with the lean-burn engine (Jpn. Pat. Appin. KOKAI Publication No. 2013-063438, Jpn. Pat. Appin. KOKAI Publication No. 2012-035253, Jpn. Pat. Appin. KOKAI Publication No. 2008-100230, Jpn. Pat. Appin. KOKAI Publication No. 2003-245523, Jpn. Pat. Appin. KOKAI Publication No. 2003-210988, Jpn. Pat. Appin. KOKAI Publication No. 2002-364338, Jpn. Pat. Appin. KOKAI Publication No. 2001-252563, and Jpn. PCT National Publication No. 2007-530271).
The NSR catalyst contains a noble metal which promotes the reduction reaction of NOx and a NOx storage material which can store and release NOx. As the NOx storage materials, for example, an alkali metal element such as potassium (K), an alkaline earth metal element such as barium (Ba), a rare earth metal element such as cerium (Ce) or zirconium (Zr), or two or more of these are used.
NOx storage materials containing an alkali metal element and an alkaline earth metal element have a large NOx storage amount per unit mass. However, it is difficult to uniformly disperse the NOx storage material containing the alkali metal element and the alkaline earth metal element in the exhaust gas-purifying catalyst. For this reason, the NOx storage amount per unit mass tends to decrease when the amount of NOx storage material used is excessively increased. In addition, NOx storage materials containing a rare earth metal element have a lower ability to adsorb NOx in the exhaust gas as compared with the NOx storage materials containing the alkali metal element and the alkaline earth metal element, and it is difficult to achieve a sufficient NOx storage amount.
Furthermore, the temperature of exhaust gas emitted from the lean-burn engine is relatively low. Hence, the NSR catalyst is required to realize excellent NOx storage ability in a relatively low temperature region.